selinux-util: increase log severity

`log_enforcing()` and `log_enforcing_errno()` are only used for important messages, which describe failures in enforced mode.
They are guarded either by `!mac_selinux_use()` or `!label_hnd` checks, where the latter is itself guarded by the former.
Only SELinux enabled systems print these logs.
This helps to configure a system in permissive mode, without getting surprising failures when switching to enforced mode.